Tank volume available for heat pump
Tank volume available for heat pump
(OP)
Hello, I have a question regarding matters outside my area of expertise. It regards optimization of a 10 kW domestic ground source heat pump. The volume of the radiator system is not big enough causing short cycling. An optimum addition of 200 liters has been calculated, and of course a purpose-built or ready-made tank can do the job, with 2 connections, one in the top as inlet for heated water from the heat pump, one in the bottom as outlet to the radiators. However, in the old boiler-room where the heat pump is located, sits an unused old 500 liter buffer tank with several connection possibilities. A colleague of mine suggested we use the old tank connected top-bottom, but I think the volume is too big. He then suggested that we could move the outlet pipe to a connection halfway up the tank's side, and that this through thermoclining would halve the volume available for the heat pump. Unfortunately I am no expert in fluid mechanics, but my limited knowledge tells me he is wrong. I just can't see the mechanics behind 250 liters of water in the bottom half of the tank not being circulated at all. Why wouldn't the outlet "suck" water from all directions, including from the lower parts? Thankful for some input on the problem.





RE: Tank volume available for heat pump
RE: Tank volume available for heat pump
RE: Tank volume available for heat pump
You could determine the final cycle time from:
V = [t * (Qheat source - q load)]/ 500*dT
V = buffer tank volume
t= desired cycle time
Qheat source = heat output rate
qload = rate of extraction from tank
dT = temperature rise of tank from the time when the heat source is turned off.
RE: Tank volume available for heat pump
RE: Tank volume available for heat pump
I can't get my head around my colleagues opinion that the water in the bottom half of the tank won't circulate. Can someone explain this to me?
RE: Tank volume available for heat pump
The key word here is thermocline.
When one body of water is colder than other the two masses separate with the warmer body on top.
Think of oil floating on water. The interface between the two masses is called the thermocline.
Since the cold water wants to move down and the warm water wants to move up there is no mixing.
True the outlet will want to suck water from all directions and as a result the thermocline may establish several centimeters below the outlet. but not enough to upset your friends calculations.
However if the buffer tank is well insulated there is nothing wrong with circulating all 500 litres, the only thing it will do, is to slow down your warm up time a little when the system is first started.
B.E.
RE: Tank volume available for heat pump
You could say the same thing about water in a 500-liter tank, albeit, the difference is so infinitissimal.
RE: Tank volume available for heat pump
When you hook-up the tank you want the heat pump to draw cold water from the bottom and return hot water to the top of the tank. You want the radiator loop to draw hot water from the top of the tank and return cold water to the bottom. This will maximize both storage capacity and responsiveness of your heating system. The thermostat controlling the heat pump should be on the bottom of your tank. If the house thermostat controls the heat pump then a storage tank will not fix short cycling.